The present illustrative embodiments are related to methods and systems for fine tuning, updating, revising, customizing or recalibrating color dictionaries, such as spot color dictionaries, associated with document processing systems. Embodiments will be described with reference to the four common subtractive colorants: cyan, magenta, yellow and black (CMYK). However, embodiments can readily be adapted for systems that use different, additional or fewer colorants.
Spot colors are colors of particular importance in a particular document, such as the color of a logo or product package or label or portion thereof. Traditionally, portions of documents calling for a particular spot color have been rendered with a matching ink in, for example, printing processes, such as offset printing.
Digital printing systems, such as electrophotographic or xerographic printing systems and/or ink jet printing systems, generate the perception of various colors by laying down varying amounts of a fixed set of colorants, such as cyan, magenta, yellow and black (CMYK) toners or inks, and are not usually readily adapted to printing a spot color through the application of a particular ink matching the particular spot color. Nevertheless, there is a desire to produce spot colors, or at least the perception of spot colors, using digital printing systems, in a manner that approaches the accuracy and consistency achievable through the use of specialized spot color inks.
Accordingly, document processing systems are sometimes preloaded with color or spot color dictionaries by a document processing system manufacturer. Such color dictionaries map spot color names and/or spot color definitions to specific colorant combinations. For example, a color name is associated with a color definition or target color measurement value in terms of some machine-independent color space description, such as, for example, the L*a*b* of the International Commission on Illumination (Commission Internationale d'Eclairage). The color dictionary associates a given name and definition, or target color measurement value, with a colorant recipe or combination of colorant values.
For example, a color dictionary preloaded in a particular document processing system may be based on a fleet or product line average color production behavior for a given manufacturer's printing fleet or for a particular model printer or print engine. Alternatively, a color dictionary may be based on a particular unit's color production behavior over its entire gamut and at a particular point in time and under a particular set of environmental circumstances, such as, soon after manufacture or prior to shipment to a customer.
While such dictionaries often provide good color matching and consistency, it is possible that a fleet average colorant recipe produces a perceived color that is undesirably far from the associated target or defined color. Additionally, due to wear and environmental factors, such as temperature and humidity, even a spot color recipe that produced the perception of a perfect match to a target spot color at the time of manufacture or calibration can produce a color that is perceived to be unacceptably far from the target color after the document processing system has been installed and operated over a period of time. Typically, such drifts or differences from the fleet average or median are compensated for with a global or gamut-wide document processing system recalibration. Such recalibrations generate global compensating lookup tables or derive new control element set points which tend to drive the overall color reproduction performance of the document processing system back toward some standard or fleet average. New spot color recipes may be derived from global models of system performance that are derived during these recalibrations and used to update spot color dictionaries.
However, such global recalibrations and/or system characterizations are based on measurements taken at relatively large intervals over each of the dimensions of device-dependent color space (e.g., CMYK). Therefore, it is not likely that a spot color will fall exactly on a compensating lookup table node. Accordingly, new spot color recipes are generally based on interpolations, such as tetrahedral interpolation between relatively widely spaced nodes of a lookup table. Such interpolations often assume a linear color production response. However, such assumptions do not always provide the accuracies required or desired in the production of spot colors. Increasing accuracy by increasing the resolution of compensating lookup tables is prohibitively expensive due to increased memory requirements and calibration process complexity.
Accordingly, there is a desire for methods and systems that provide improved fine tuning, customization and/or recalibration of color dictionaries, such as, for example, spot color dictionaries.